As can be seen from the aforementioned copending applications, the literature cited below and the references in the aforementioned copending applications, it is known to provide a control device for the setting (adjustment of displacement) of an axial piston pump which includes a setting piston constituted as a differential piston. The latter has a small effective surface which can be subjected to the output pressure of the pump directly and a large-diameter effective surface which can be subjected to this pump pressure under the control of a further element such as a valve member. Between the valve member and the setting piston, there can be provided a force-transmitting system which can include a bell crank lever.
The axial piston pump or pumps with which the system of the present invention can be used can be any of those described in the aforementioned copending applications or in the literature set forth below. In such an axial piston pump, the cylinder drum usually rides along a rocker or swingable slide which can be shifted to one side or another of an angular orientation in which the plane of the surface against which the cylinder drum reacts lies perpendicular to the axis of the drive shaft.
The axially displaceable pistons of this drum can react against the surface coupled with the drive shaft so that, depending upon the side to which the drum is swung from its neutral position in which its axis of rotation coincides with the axis of the drive shaft, one or the other port will serve as the outlet port and the angle to which the drum is swung from the neutral position will determine the displacement per revolution of the pump.
With conventional control device of the aforedescribed type, which are generally designed to maintain a constant output product of the pump for a given setting thereof, i.e. a constant product of the discharge pressure and displacement volume per revolution, the fulcrum of the bell crank lever is not fixed but is determined by the position of a pressure-measuring piston.
More particularly, the bell crank lever is fulcrumed on the pressure-measuring piston and is provided with a slit in which a pin for an element connected to the setting piston is slideable (see German open application--Offenlegungsschrift--DT-OS No. 1 653 385).
This construction has been found to be particularly unreliable and weak, especially when the machine is subjected to vibration, since the control mechanism is readily degenerated. The friction forces result in erroneous corrections of the position of the setting element and the lateral forces upon the pressure-measuring piston (control piston) give rise to an excessive wear so that increased leakage and power losses result. A further disadvantage of this system is that for given controller dimensions, the hyperbolic law of force transmission between the controlled and controlling elements can only be adjusted as to one parameter so that an optimum setting of the controller is not always possible.
In addition, the aforedescribed prior-art configuration of a control device is not suitable for setting or control systems in which the setting piston must be afforded a large piston stroke. This is especially the case in pumps in which the displaceable member is a swingable slide. In this case, difficulties are encountered because a spring sufficient to accommodate the desired stroke of the setting piston must be of large volume and hence occupy considerable space. It is also expensive to provide means for avoiding the buckling of the spring.
In yet another conventional system for the control of axial piston machines (see German open application--Offenlegungsschrift--DT-OS No. 2 003 774) at least approximately parallel to the setting piston there is provided a lever such that one end of the lever is articulated with the setting piston and between the other end of the lever and the setting piston, a spring is provided under prestress. The lever presses with its lateral face against the control slide. In this embodiment as well, the articulation or fulcrum of the lever has a changeable location and there is a disadvantage in the fact that the lever shifts the setting piston which applies lateral forces upon the control slide.
In German open application--Offenlegungsschrift--DT-OS No. 2 017 656, moreover, there is described a control device for the adjustment of an axial piston pump which includes a setting body and a setting piston connected with this setting body and formed as a differential piston. It is such a system that the present invention intends to improve upon as noted below. The differential piston has a small effective surface which is subjected directly to the output pressure of the pump and a large effective surface which is pressurized with the output pressure of the pump via a control element such as a valve member. A control slide is used in this system as well and a bell crank lever is provided whose fulcrum is fixed on the housing and a force-transmitting member is connected with the setting piston and has a pressure body which lies perpendicular to the axis of the setting piston and bears against one shank or arm of the bell crank lever. In this system as well disadvantages are encountered since the provision of the control piston in the setting piston is extremely expensive and the arrangement subjects the control piston and the setting piston to lateral forces which are extremely disadvantageous.
Still another arrangement using a lever system provides for the transmission of force by the lever system with variable transmission ratio. In this case, the setting piston is connected with a slide block which rests against the setting part on the one hand and against a lever on the other, whereby an end of the lever is supported by a piston subjected to the pump pressure. The second end bears against a spring-supported control slide (German printed application--Auslegeschrift--DT-AS No. 1 044 620). In this case as well, the fulcrum of the arm lever is movable.
In a kinematic reversal to the foregoing (see German open application--Offenlegungsschrift--DT-OS No. 2 003 774) the lever has one end fixedly hinged while the opposite end bears against a spring fixed in the housing. The control valve is provided in the piston rod of a piston which is shiftable approximately parallel to the lever. In this case disadvantages are also encountered which appear to result from the need to accommodate the control piston in the piston rod. A discussion of this point can be found in German Offenlegungsschrift DT-OS No. 2 017 656. See also the following German open applications--Offenlegungsschrifts--DT-OS No. 1 498 288 and No. 1 267 092.
Reference may also be had to German open applications--Offenlegungsschrifts--DT-OS No. 1 400 630 and DT-OS No. 1 425 756 which describe other control devices in which a lever system with a variable transmission ratio is employed. These systems are provided, in a manner not unlike that of the German printed application--Auslegeschrift--DT-AS No. 1 044 620, with a lever whose fulcrum is shiftable and having one end at which a spring acts. Here, however, the prestress and hence the reaction force of the spring is dependent upon the position of the setting piston and varies therewith. In this system all of the aforedescribed disadvantages of lever fulcrums which vary as to location (nonfixed lever fulcra) are encountered.